In many diseases, a body cavity, such as a passageway or channel (e.g. blood vessel, esophagus, bronchus, etc.) through which a body fluid or other substance (e.g., air in bronchus) flows, may collapse or be narrowed to be substantially restricted. A passageway can be so restricted that fluid flow in the passageway is diminished or blocked. For example, in coronary artery disease, the coronary artery of a mammal is narrowed or restricted such that blood flow through the artery is diminished. Even after balloon angioplasty procedures, such as percutaneous transluminal angioplasty, in which a blood vessel is dilated by means of a balloon catheter to flatten plaque against the artery wall, in many cases, restenosis occurs soon afterwards and the vessel becomes restricted again. Following percutaneous balloon angioplasty, the arterial wall sometimes develops tears. As a result, flaps of the inner layer of the arterial wall may hang loosely in the blood vessel, causing obstruction to blood flow and requiring emergency bypass surgery. There is a need for a means to maintain patency of collapsing body cavities or blood vessels and to prevent the renarrowing of the vessel after angioplasty.
Stents can be used to provide mechanical support to maintain the patency of blood vessels. Similarly, the patency of body cavities and passageways such as urethra, bile duct, esophagus, ureters,aorta, etc., can also be maintained by stents. Stents of various shapes and designs have been utilized for such purposes. For example, U.S. Pat. No. 4,886,062 (Wiktor) discloses an intravascular radially expandable stent and method of implantment thereof. The stent disclosed by Wiktor comprises a wire wound into a continuous helix along the length of the stent. The stent is made of a low-memory metal and is radially expanded by inflating a catheter balloon, which exerts a force on the stent. U.S. Pat. No. 4,969,458 (Wiktor) also discloses a stent made of low-memory metal, expandable radially to a larger diameter by the action of inflation of a balloon. U.S. Pat. No. 5,133,731 (Wiktor) discloses yet another stent made of low-memory material. The stent has a cylindrical body coiled from a generally continuous wire with a deformable zig-zag structure. Means for preventing the stent's body from stretching along its longitudinal axis are also present in the stent.
Because stents made with low-memory materials require mechanical force to expand the stent, such as a force exerted by the inflation of a balloon, their use can result in trauma to the body caused by the imprecise control of the expansion of the stent or the balloon. Moreover, stents made with low memory material may have a tendency to be compressed into a smaller diameter by the radial, inwardly directed force exerted by the body tissue on the stent. Self-expanding stents have been developed to obviate the use of externally applied mechanical force for their expansion. For example, U.S. Pat. No. 4,830,003 (Wolff) discloses a cylindrical stent for preventing arterial closure and restenosis. The stent is made of biocompatible metal wires welded together in pairs at alternate ends with each pair of wires bent into a V-section. The stent is compressed and loaded into an outer catheter, positioned in a selected location and released for self-expansion by an inner catheter. U.S. Pat. No. 5,104,404 (Wolff) discloses an articulated stent made up of a number of individual stent segments. A number of wires are welded together to form a segment, and adjacent stent segments are connected together to provide a hinge action. In a similar fashion, U.S. Pat. No. 5,035,706 (Gianturco) discloses a self-expanding stent formed of stainless steel wire arranged in a closed zig-zag configuration including an endless series of straight sections joined at their ends by bends. The bends of at least one end of the stent are formed into eyes for connection with eyes at one end of a similarly constructed stent. The stents are compressible into reduced diameter size for insertion into and removal from a body passageway.
Because self-expanding stents, such as those made from stainless steel, once expanded, cannot be deformed unless an external force is applied thereto, such stents generally cannot be removed from the body cavity once they are deployed therein. Therefore, stents that can be brought back to a smaller shape and size after expansion within a body cavity have been developed to enable removal after deployment. For example, U.S. Pat. No. 5,037,427 (Harada et al.) discloses a method of implanting a stent and removing same from a tubular organ. The stent is formed of a two-way shape-memory alloy and expands or shrinks in the radial direction, in accordance with changes in temperature. Also, U.S. Pat. No. 5,147,370 (McNamara et al.) discloses a coil stent constructed from a nitinol alloy. However, because such stents expand by the heat of the body, there can be a risk that such a stent would expand before it is properly deployed or positioned in the desired location.
U.S. Pat. No. 5,026,377 (Burton) discloses a stent placement instrument and method for deployment or retraction of a self-expanding stent in a body canal. The instrument comprises an elongated tubular outer sleeve having disposed therein an elongated core which is movable relative to the sleeve and has a grip member for releasably holding a self-expanding stent within the outer sleeve. U.S. Pat. No. 5,078,720 (Burton) discloses yet another stent placement instrument and method for the placement of a self-expanding stent in a body canal. The instrument comprises an elongated inner tube having an outer tube disposed along its axis for carrying and retaining a self-expanding stent and an arrangement for releasing the stent, in combination with at least one of: (a) a location member for positioning and fixing the instrument so that the stent is released at a desired location in the body canal, and (b) a member for releasing the stent in a retrograde manner. The stents disclosed by Burton in the two patents are wire-mesh-type stents.